Strange readings

RFGumby · 4598

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Offline RFGumby

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on: June 01, 2010, 03:34:22 AM
Hi Guys-

      Long time BH member and user, but haven't posted for years since the old site not run by Doc.
The last things I've built were highly modded FP2's and a Seduction in a copper chassis and a remote matching PS.

       I recently built a Crack given to me by a friend, and upon testing noticed something I had thought before to be unprecedented in all my engineering experience.  Maybe someone can shed light on this.  During voltage readings, the voltages at TP1 and TP5 were around 80 volts.  Cool.  Then the strange stuff happens...

With the supplied 6080 tube in place, the socket lugs B1 and B3 read 26 volts.

With a 6AS7G tube I had in stock in place, the voltages at B1 and B3 read 65 volts.

The problem with me lies int he fact that the TP1 and TP5 barrier strip lugs are hardwired to B1 and B3 respectively.  School teaches you that two different readings at opposite ends of the same wire are not possible, and this is the first time I've seen it in a whole bunch of years doing electronics.

The kit works fine, the only problem is a DC offset type "pop" at the bottom end of the volume pot, on both volume going up and down.  The only variation from stock is the use of a 6DJ8 tube for the driver and the according heater change to accommodate this.

Scott


Offline Doc B.

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Reply #1 on: June 01, 2010, 04:41:52 AM
Off the top of my head, I would guess it is maybe some weird interaction of your meter and the cathode bypass caps. Try using a different ground point to attach the black test lead and see if the results change.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline Paul Birkeland

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Reply #2 on: June 01, 2010, 08:37:13 AM
I'd imagine you mean B1 and B4 don't measure the same as T1 and T5?

I might measure the resistances between T1 and its connection to the B socket, as well as T5 and its connection to the B socket. 


Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline RFGumby

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Reply #3 on: June 01, 2010, 09:56:18 AM
Hi guys, thanks for the assistance.

    All functions normally in operation, so it's a little strange.   I went back and tried the measurements from different ground points and they cam up the same. 

Paul- "I'd imagine you mean B1 and B4 don't measure the same as T1 and T5?"

Yes, sorry for the typo. You are correct.  I also checked the resistance between T1/B1, and T5/B4, and they check as wire should, 0 ohms (lol).  I think I already had... I don't post questions much as I do lots of this type of mod and DIY work, and this is a rather simple circuit.  The part that really blew me away was with the original 6080, the voltages at B1 and B4 were only 24 volts.  So before trying a different tube I went and checked through all the assembly and didn't see any issues.  I then tried the 6AS7 and was comforted to see the voltages at B1 and B4 were at least closer at 65vdc.

Strange but true.  Huh.

Scott

Scott